Nick Fury (Samuel L Jackson), director of the espionage agency S.H.I.E.L.D., arrives at a remote research facility during an evacuation. The tesseract, an energy source of unknown potential, has activated and opened a portal through space, from which the exiled Norse god Loki (Tom Hiddleston) steps through. Loki takes the tesseract, and uses his abilities to control the minds of several S.H.I.E.L.D. personnel including agent Clint Barton (Jeremy Renner), and physicist consultant Dr. Erik Selvig (Stellan Skarsgard) in order to aid in his getaway.
In response to the attack, Fury reactivates the Avengers Initiative. Agent Natasha Romanoff (Scarlett Johannson) is sent to India to recruit Dr. Bruce Banner (Mark Ruffalo), while Agent Philip Coulson (Clark Gregg), approaches Tony Stark (Robert Downey Jr.) and requests that he review Selvig's research. Fury himself approaches Steve Rogers (Chris Evans) with an assignment to retrieve the tesseract from Loki. During his exile, Loki encountered the Other (Alexis Denisof), an alien conqueror who in exchange for the Tesseract offers Loki an army of the alien race the Chitauri in order for him to subjugate Earth.
Rogers, Stark and Romanoff travel to Germany to apprehend Loki, who is recovering iridium needed to stabilize the Tesseract's power and demanding that the civilians kneel before him. After a battle with Captain America, Loki surrenders and is escorted back to a S.H.I.E.L.D. plane. However, Thor (Chris Hemsworth), Loki's adoptive brother and the Norse god of thunder, arrives and attempts to free Loki to reason with him. Stark and Rogers confront Thor, and Loki is eventually returned to the Helicarrier, a high-tech, flying aircraft carrier, and placed in a cell designed to hold The Hulk.
The Avengers are divided, both over how to approach Loki and the revelation that S.H.I.E.L.D. plan to harness the tesseract's power to develop weapons. Fury admits that the events in New Mexico a year before made S.H.I.E.L.D. aware of extraterrestrial life, some of whom may see Earth as a target. The decision was made to produce weapons with the tesseract as a means of deterrence. As the group argues, Barton and Loki’s other possessed agents attack the Helicarrier, disabling its engines in flight. As Stark and Rogers attempt to restart the damaged engines, Banner transforms into the Hulk, despite Romanoff's best efforts to calm him down, and runs amok inside the ship, fighting Thor. During a fight with Barton, Romanoff discovers that a blow to the head — powerful enough to knock Barton unconscious — is enough to break Loki's mind control. Loki escapes, killing Agent Coulson as he does so, and Thor and the Hulk are each ejected from the ship......

Nick Fury (Samuel L Jackson), director of the espionage agency S.H.I.E.L.D., arrives at a remote research facility during an evacuation. The tesseract, an energy source of unknown potential, has activated and opened a portal through space, from which the exiled Norse god Loki (Tom Hiddleston) steps through. Loki takes the tesseract, and uses his abilities to control the minds of several S.H.I.E.L.D. personnel including agent Clint Barton (Jeremy Renner), and physicist consultant Dr. Erik Selvig (Stellan Skarsgard) in order to aid in his getaway.
In response to the attack, Fury reactivates the Avengers Initiative. Agent Natasha Romanoff (Scarlett Johannson) is sent to India to recruit Dr. Bruce Banner (Mark Ruffalo), while Agent Philip Coulson (Clark Gregg), approaches Tony Stark (Robert Downey Jr.) and requests that he review Selvig's research. Fury himself approaches Steve Rogers (Chris Evans) with an assignment to retrieve the tesseract from Loki. During his exile, Loki encountered the Other (Alexis Denisof), an alien conqueror who in exchange for the Tesseract offers Loki an army of the alien race the Chitauri in order for him to subjugate Earth.
Rogers, Stark and Romanoff travel to Germany to apprehend Loki, who is recovering iridium needed to stabilize the Tesseract's power and demanding that the civilians kneel before him. After a battle with Captain America, Loki surrenders and is escorted back to a S.H.I.E.L.D. plane. However, Thor (Chris Hemsworth), Loki's adoptive brother and the Norse god of thunder, arrives and attempts to free Loki to reason with him. Stark and Rogers confront Thor, and Loki is eventually returned to the Helicarrier, a high-tech, flying aircraft carrier, and placed in a cell designed to hold The Hulk.
The Avengers are divided, both over how to approach Loki and the revelation that S.H.I.E.L.D. plan to harness the tesseract's power to develop weapons. Fury admits that the events in New Mexico a year before made S.H.I.E.L.D. aware of extraterrestrial life, some of whom may see Earth as a target. The decision was made to produce weapons with the tesseract as a means of deterrence. As the group argues, Barton and Loki’s other possessed agents attack the Helicarrier, disabling its engines in flight. As Stark and Rogers attempt to restart the damaged engines, Banner transforms into the Hulk, despite Romanoff's best efforts to calm him down, and runs amok inside the ship, fighting Thor. During a fight with Barton, Romanoff discovers that a blow to the head — powerful enough to knock Barton unconscious — is enough to break Loki's mind control. Loki escapes, killing Agent Coulson as he does so, and Thor and the Hulk are each ejected from the ship......